Commit 73fa7547 authored by Rich Felker's avatar Rich Felker Committed by Christian Brauner
Browse files

vfs: add RWF_NOAPPEND flag for pwritev2 



The pwrite function, originally defined by POSIX (thus the "p"), is
defined to ignore O_APPEND and write at the offset passed as its
argument. However, historically Linux honored O_APPEND if set and
ignored the offset. This cannot be changed due to stability policy,
but is documented in the man page as a bug.

Now that there's a pwritev2 syscall providing a superset of the pwrite
functionality that has a flags argument, the conforming behavior can
be offered to userspace via a new flag. Since pwritev2 checks flag
validity (in kiocb_set_rw_flags) and reports unknown ones with
EOPNOTSUPP, callers will not get wrong behavior on old kernels that
don't support the new flag; the error is reported and the caller can
decide how to handle it.

Signed-off-by: default avatarRich Felker <dalias@libc.org>
Link: https://lore.kernel.org/r/20200831153207.GO3265@brightrain.aerifal.cx


Reviewed-by: default avatarJann Horn <jannh@google.com>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 9e3f1c59

include/linux/fs.h

+8 −0
Original line number Diff line number Diff line
@@ -3335,6 +3335,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags) 
		return 0;

	if (unlikely(flags & ~RWF_SUPPORTED))

		return -EOPNOTSUPP;

	if (unlikely((flags & RWF_APPEND) && (flags & RWF_NOAPPEND)))

		return -EINVAL;



	if (flags & RWF_NOWAIT) {

		if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
@@ -3345,6 +3347,12 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags) 
	if (flags & RWF_SYNC)

		kiocb_flags |= IOCB_DSYNC;



	if ((flags & RWF_NOAPPEND) && (ki->ki_flags & IOCB_APPEND)) {

		if (IS_APPEND(file_inode(ki->ki_filp)))

			return -EPERM;

		ki->ki_flags &= ~IOCB_APPEND;

	}



	ki->ki_flags |= kiocb_flags;

	return 0;

}

include/uapi/linux/fs.h

+4 −1
Original line number Diff line number Diff line
@@ -301,9 +301,12 @@ typedef int __bitwise __kernel_rwf_t; 
/* per-IO O_APPEND */

#define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)



/* per-IO negation of O_APPEND */

#define RWF_NOAPPEND	((__force __kernel_rwf_t)0x00000020)



/* mask of flags supported by the kernel */

#define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\

			 RWF_APPEND)

			 RWF_APPEND | RWF_NOAPPEND)



/* Pagemap ioctl */

#define PAGEMAP_SCAN	_IOWR('f', 16, struct pm_scan_arg)